1. Field of the Invention
The present invention relates to methods of measuring electrical signals and more particularly to a method of measuring the duration of a discontinuous signal which consists of a group of pulses with the duration of the gap between individual pulses in the same group being smaller than the time between groups.
2. Description of the Prior Art
In some electronic instruments, it is necessary to measure the time during which a discontinuous signal is present. If the signal contains gaps which should be ignored in the time-duration measurement, then problems arise which are not solved by standard prior-art devices for simply measuring time intervals.
Consider the case, for example, where the input signal has been converted to digital form and is composed of groups of pulses, with the separation between individual pulses in the same group being distinctly smaller than the separation between pulse groups. In this case, it is desired to measure the time during which the pulse group is present, starting at the leading edge of the first pulse and ending at the trailing edge of the last pulse within the same group, without regard to the relatively small gaps between the individual pulses within that group.
One method for measuring this time interval is to pass the discontinuous input pulse through an analog filter with a response time which is sufficiently slow so that a subsequent discriminator does not detect the pulse gaps. Then the width of the discriminator output pulse, which becomes a smoothed version of the original discontinuous input signal, can be measured by well-known techniques.
However, the required response time of the analog filter depends upon the duration of the gaps between the pulses in the same pulse group. If these gaps are variable, then the filter response time must often be repeatedly adjusted to compensate for this variation. Furthermore, in the event that the total width of the pulse group is not much larger than the largest gap between the pulses within the same group, then the response of the analog filter must become comparable to the duration of the pulse group itself. In that case the analog filter may introduce time-delay errors which are intolerably large.
A second common method for handling discontinuous signals of this nature is applicable whenever the pulses within the same pulse group always occur at a known periodic rate. In that case one may simply count the number of pulses present in a given group and compute the time duration of the group from the known pulse period. However, in the general case where the pulses are not periodic or the pulse period is unknown, this method cannot be used.